U.S. Pat. No. 3,962,866 discloses a two-level lambda-control method wherein the difference between the signals of a probe forward of the catalytic converter and of a probe rearward of the catalytic converter is formed as a value for evaluating the performance loss of the converter. The poorer the conversion of the converter, the greater is the amplitude of the signal of the rearward probe. For this reason, the small difference between the signals from the two probes becomes ever smaller with increasing performance loss of the converter. A warning signal is emitted when this value drops below a threshold value. In order that the investigated value is not too dependent upon the operating point at which the engine is operated, it is advantageous to determine this value only during steady-state operation of the engine.
The probes used in the method just described and in the arrangement corresponding thereto are oxygen probes. In the entire further description, the assumption is made that such probes are used forward and rearward of the catalytic converter. However, any other probe can be utilized which is in the position to indicate the quantity of one component or several components in the exhaust gas of the internal combustion engine.
It has been established that the method described above and the arrangement corresponding thereto are very susceptible to disturbances and therefore permit only a coarse evaluation of the performance loss of the catalytic converter.